1. Field of the Invention
The present invention relates to an austenitic stainless steel superior not only in the workability but also in the corrosion resistance for use for the material of, such as, absorption towers, cooling towers, pumps, vessels and so on, to be employed in an environment of high temperature concentrated sulfuric acid in sulfuric acid industry, in particular, for dealing with sulfuric acid of a concentration of 90-100% at a temperature of up to 240.degree. C.
2. Description of the Related Art
Sulfuric acid has in general a high corrosive property for metals. Such attack of metals by sulfuric acid is quite considerable especially at medium concentrations of sulfuric acid from about 10 to about 80%. This is attributed mainly to the fact that such medium concentration sulfuric acid is a non-oxidative acid. Existing materials capable of withstanding such sulfuric acid environment are quite limited and may be exemplified, for use at temperatures below 100.degree. C., by lead and some of Ni alloys, such as, Hastelloy B and C276 (trade names).
It is known, on the other hand, that oxidizing conditions take place when sulfuric acid is concentrated up to 90% or higher. For such highly concentrated sulfuric acid, some metals which do not withstand a medium concentration sulfuric acid may become tolerable for use. For example, mild steel has a better corrosion resistance against a highly concentrated sulfuric caid of 98% at lower temperatures, due to formation of an anti-corrosive protective layer of FeSO.sub.4 over the entire surface of the steel, so that it finds practical use for such highly concentrated sulfuric acid at room temperature (at around 20.degree. C.).
At higher temperatures up to 240.degree. C. to be encountered in sulfuric acid industry, the attacking action of sulfuric acid becomes violent. At such a high temperature, the protective FeSO.sub.4 coating layer of milde steel will tend to dissolve in the highly concentrated sulfuric acid to destroy the anti-corrosive passive layer, resulting in destruction of corrosion resistance of mild steel.
Usual austenitic steels, various ferrite steels and nickel alloys exhibit poor corrosion resistance in such highly concentrated high temperature sulfuric acid and even lead and Ni-alloys, such as Hastelloy B and C-276 (trade names), exihibiting relatively high corrosion resistance in medium concentration sulfuric acid become less resistant at high temperatures to highly concentrated sulfuric acid.
No material has been found up to date, which has sufficient resistance in such environment and which is applicable practically for various installations and instruments in the sulfuric acid industry. However, it has been known, that high Si cast iron (containing more than 14% of Si) exhibits relatively superior corrosion resistance in high concentration sulfuric acid at lower temperatures (below about 120.degree. C.). It has been assumed that Si contributes effectively to the development of anti-corrosive property effectively. It has recently been reported that ferritic stainless steels having high content of Cr exhibit also relatively better corrosion resistance in such an environment. This suggests that Cr may contribute to the development of corrosion resistance effectively and that the content of Ni which is assumed to have a negative effect on the development of anti-corrosive property is low.
However, these steels have poor mechanical workability and, in particular, high Si cast iron is scarcely able to be subject to mechanical working and welding, so that it finds no practical use for large sized installations and instruments. Thus, in the practice, large sized installations to be employed in an environment of highly concentrated sulfuric acid of above 90% at a temperature of up to 120.degree. C., such as, absorption towers and so on, are lined internally with acid-resistant bricks.
Such internal lining suffers from the problems such as follows:
The binder material employed to fill up the interstices between the adjoining acid-resistant bricks will be damaged during the course of long-term operation by the highly concentrated sulfuric acid, which may cause leakage of sulfuric acid, so that it is necessary to incorporate an overhauling of the entire installation at intervals of a few years. Such a damage of the binding material will markedly be accelerated under the conditions with which the present invention deals, namely, sulfuric acid of a concentration of above 90% and a temperature of up to 240.degree. C. and the durability of the brick will also promotively be damaged.
Also, high Cr ferritic stainless steels which have relatively better corrosion resistance as compared with other materials will suffer from corrosion attack under the condition mentioned above and will be subject to a corrosion rate exceeding over the critical allowable value of 0.1 g/cm.sup.2.hr for the practical use. This is because that the content of Cr is not allowed to reach the amount necessary for attaining sufficient corrosion resistance under the condition mentioned above, namely, over 35%, in order to maintain a tolerable workability. When the content of Cr is increased, the resulting high Cr ferritic stainless steel becomes brittle and mechanical working, such as, pressing and rolling, becomes difficult. Upon welding such a high Cr ferritic stainless steel, incorporation of additional technical measures, such as, preheating, after-heating and so on, is necessary for avoiding the hardening of the material around the welded portion, resulting in a considerable increase in the costs for manufacturing and overhauling such installations, as compared with materials of austenitic stainless steels.
As for high Si cast iron, the problem that a mechanical working and welding will scarcely be permitted due to the brittleness of the high Si cast iron is left unsolved.
Under the circumstances of the stand of the technique described above, it is contemplated by the present invention to provide a novel austenitic stainless steel which resolves the disadvantage of poor corrosion resistance associated with the conventional material in the environment of highly concentrated high temperature sulfuric acid and which permits welding and mechanical working without problem.